This invention relates to a communications cable suitable for plenum, riser, and other applications in building structures and, more particularly, relates to an improved construction for such a cable which is capable of meeting burn requirements while at the same time being able to transmit high frequency signals in a local area network.
It is common practice to route communication cables and the like for computers, data devices, and alarm systems through plenums in building constructions. If a fire occurs in a building which includes plenums or risers, however, the non-fire retardant plenum construction would enable the fire to spread very rapidly throughout the entire building. Fire could travel along cables installed in the plenum, and smoke originating in the plenum could be conveyed to adjacent areas of the building.
A non-plenum rated cable sheath system, which encloses a core of insulated copper conductors, and which comprises only a conventional plastic jacket, may not exhibit acceptable flame spread and smoke generation properties. As the temperature in such a cable rises due to a fire, charring of the jacket material may occur. If the jacket ruptures, the interior of the jacket and the insulation are exposed to elevated temperatures. Flammable gases can be generated, propagating flame and generating smoke.
Generally, the National Electrical Code requires that power-limited cables in plenums be enclosed in metal conduits. This is obviously a very expensive construction due to the cost of materials and labor involved in running conduit or the like through plenums. The National Electrical Code does, however, permit certain exceptions to the requirements so long as such cables for plenum use are tested and approved by an independent testing laboratory, such as the Underwriters Laboratory, as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production characteristics of cable are measured per specification UL-910 plenum burn analysis.
With plenum cables, in addition to concerns about flammability and smoke production, the cables must also, of course, have suitable electrical characteristics for the signals intended to be carried by the cables. There are various categories of cable, such as Category 3, Category 4, Category 5, etc., with increasing numbers referring to enhanced or higher frequency electrical transmission capabilities. With Category 5, for example, extremely good electrical parameters are required, including low attenuation, structural return loss, and cross-talk values for frequencies up to 100 MHz. Unfortunately, cable materials which generally have the requisite resistance to flammability and smoke production also result in electrical parameters for the cable generally not suitable for the higher transmission rates, such as a Category 5 cable. Specifically, in the case of cables intended for Category 5, the cable core, in addition to passing the plenum burn test UL-910, must also pass physical property testing provided by the specification requirements UL-444, as well as meet Category 5 electrical requirements such as provided in Electronic Industries Association specification TIA-568A.
Currently, a cable construction which is available and which meets these requirements is provided in a configuration which includes fluorinated ethylene propylene (FEP) as insulation, with a low smoke (LS) polyvinyl chloride (PVC) jacket. Such a cable construction meets the 100 MHz frequency operation requirements, and it has been demonstrated that such a cable construction can be suitable for operation at 155 Megabits or 150 MHz. Unfortunately, FEP is in short supply. Given the manufacturing capacity of FEP producers, only enough FEP is currently produced to meet approximately 50% of the demand for the volume of material required to construct high-category cables. Although it could be expected that the supply of FEP will continue to increase, it is apparent that the available quantity of FEP may not meet the demand for the material for use in plenum cables as the market is projected to increase at a rate of 30% per year through 1999.
Current riser cables utilize a foam/skin insulation (referred to hereafter as xe2x80x9cF/Sxe2x80x9d). The insulation material construction is a foamed, high density polyethylene (HDPE)/polyvinyl chloride (PVC) skin composite. A jacketed and shielded cable of these insulation cores meets Category 3 electrical and the CMR burn requirements. However, developing a Category 5 cable is very difficult due to the extreme electrical parameters necessary; i.e., attenuation, structural return loss, and cross-talk values to 100 Mhz. Furthermore, this core must pass elevated temperature attenuation requirements at 40xc2x0 C. and 60xc2x0 C. The above-mentioned insulation composite with a PVC skin will not pass the elevated temperature attenuation requirements because the dielectric constant of PVC increases with temperature.
It is an object of this invention to provide a cable construction suitable for high frequency electrical applications while at the same time being resistant to burning.
It is a more specific object of this invention to provide a cable design that meets Category 5 or higher electrical parameters, including elevated temperature attenuation requirements, while at the same time satisfying the burn rating standards for plenum cable.
It is an additional object of this invention to provide a cable construction which meets the electrical and burn rating requirements and additionally meets various physical requirements such as cold bend, room temperature and aged tensile strength, elongation, and the like, required for plenum cables.
It is another object of this invention to provide such a cable construction meeting the above requirements, which does not utilize fluorinated ethylene propylene material (FEP), and which is suitable for operation up to 155 Megabits or 150 MHz.
It is an additional specific objective of this invention to provide a novel material for a core wrap wherein the core wrap meets burn requirements without additional burn protection jackets or layers.
Briefly, in accordance with one embodiment of the invention, a riser and plenum rated cable construction includes a plurality of twisted wire pairs utilizing a polyolefin primary insulation material, an outer jacket for the cable construction formed of a thermoplastic halogenated polymer, and a inner jacket or core wrap separating the twisted wire pairs from the halogenated polymer.